This invention relates to the vapor phase dehydration of .alpha.-alkylbenzyl alcohols and substituted analogues thereof to form styrene and substituted styrenes.
Dehydration of alcohols to their corresponding unsaturated structural compounds is well known in the art. Dehydration techniques are not generally employed in the manufacture of styrene and many homologues thereof because standard dehydrogenation of ethyl benzene is considered to be a more economic route. In addition, styrenes produced by conventional dehydration techniques often contain enough ethyl benzene and other impurities to require extensive purification.
It is characteristic of standard dehydrogenation techniques employed in the production of styrene that fairly large quantities of unreacted ethyl benzene be present in the styrene fraction. Such quantities of ethyl benzene in the styrene fraction are substantial enough to cause loss of properties in polymers of such styrene fractions. Furthermore, due to the closeness of the boiling points of styrene and ethyl benzene, removal of ethyl benzene by distillation is expensive.
Moreover, normal dehydrogenation of many substituted ethyl benzenes, particularly the tertiary alkyl substituted ethyl benzenes, destroys or alters the substituted group. For example, dehydrogenation of ar-(t-alkyl)-ethyl benzene to form their corresponding styrenes usually results in rupture and/or loss of the t-alkyl group as well as ethyl group dehydrogenation.
Attempts to prepare ar-(t-alkyl)styrenes by conventional dehydration of the corresponding ar-(t-alkyl)-.alpha.-methylbenzyl alcohols have not been satisfactory due to the formation of other byproducts and the rupture of the t-alkyl group which frequently accompanies dehydration. As a result of this rupture, appreciable quantities of ethyl benzene and under certain conditions, diolefinically unsaturated aromatic monomers are formed in addition to the desired ar-(t-alkyl)styrene. These diolefinically unsaturated aromatic monomers, e.g., ar-(i-propenyl)styrene in dehydration of ar-(t-butyl)-.alpha.-methylbenzyl alcohol, are very difficult to separate from the desired ar-(t-alkyl)styrene. During polymerization of the ar-(t-alkyl)styrene monomer, the diolefinically unsaturated aromatic monomer acts as a crosslinking agent thereby producing a substantially crosslinked styrene polymer which is insoluble in many organic solvents such as toluene and benzene. This lack of solubility is undesirable in many applications employing such styrene polymers.
Conventional dehydration techniques for preparing styrene and substituted styrenes are not completely satisfactory in that substantial amounts of ethyl benzene and other difficult to separate impurities often remain or are produced. Such difficulties have been pointed out in prior publications such as U.S. Pat. Nos. 2,399,395 and 3,442,963.
Therefore, it would be highly desirable to provide a new, improved technique for producing styrene and substituted styrenes in high yield which contain little or no ethyl benzene and other impurities, particularly diolefinically unsaturated aromatic monomers.